copyright 2010, john wiley & sons, inc. chapter 9 nervous tissue
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Copyright 2010, John Wiley & Sons, Inc.
Chapter 9
Nervous Tissue
Copyright 2010, John Wiley & Sons, Inc.
End of Chapter 9
Copyright 2010 John Wiley & Sons, Inc.All rights reserved. Reproduction or translation of this work beyond that permitted in section 117 of the 1976 United States Copyright Act without express permission of the copyright owner is unlawful. Request for further information should be addressed to the Permission Department, John Wiley & Sons, Inc. The purchaser may make back-up copies for his/her own use only and not for distribution or resale. The Publishers assumes no responsibility for errors, omissions, or damages caused by the use of theses programs or from the use of the information herein.
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Structures of the Nervous System Brain: neurons enclosed within skull Spinal cord: connects to brain and enclosed
within spinal cavity Nerves: bundles of many axons of neurons
Cranial nerves (12 pairs) emerge from brain Spinal nerves (31 pairs) emerge from spinal cord
Ganglia: groups of neuron cell bodies located outside of brain and spinal cord
Enteric plexuses: networks in digestive tract Sensory receptors: monitor changes in internal
or external environments
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Structures of the Nervous System
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Functions of the Nervous System Sensory receptors and sensory nerves
Carry information into brain and spinal cord Integration: information processing
Perception = awareness of sensory input Analyzing and storing information to help lead to
appropriate responses Motor activity: efferent nerves
Signals to muscles and glands (effectors)
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Organization of the Nervous System Central Nervous System (CNS)
Brain and spinal cord Peripheral Nervous System (PNS)
All nervous system structures outside of the CNS
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Histology of the Nervous System Neurons
Can respond to stimuli and convert stimuli to electrical signals (nerve impulses) that travel along neurons
Neuroglia cells: support, nourish and protect neurons Neuroglia critical for homeostasis of interstitial fluid
around neurons
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Neuronal Structure Cell body: nucleus, cytoplasm with typical
organelles Dendrites: highly branched structures that
carry impulses to the cell body Axon: conducts away from cell body toward
another neuron, muscle or gland Emerges at cone-shaped axon hillock
Axon terminals: contain synaptic vesicles that can release neurotransmitters
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Neuronal Structure
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Structural Classes of Neurons Multipolar
Have several or many dendrites and one axon Most common type in brain and spinal cord
Bipolar Have one dendrite and one axon Example: in retina of eye and inner ear
Unipolar Have fused dendrite and axon Sensory neurons of spinal nerves
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Functional Classes of Neurons Sensory (afferent)
Convey impulses into CAN (brain or spinal cord) Motor (efferent)
Convey impulses from brain or spinal cord out through the PNS to effectors (muscles or glands)
Interneurons (association neurons) Most are within the CNS Transmit impulses between neurons, such as
between sensory and motor neurons
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Neuroglia Cells smaller but much more numerous than
neurons Can multiply and divide and fill in brain areas Gliomas: brain tumors derived from neuroglia Functions
Do not conduct nerve impulses Do support, nourish and protect neurons
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Neuroglia Astrocytes: help form blood brain barrier Oligodendrocytes: produce myelin in CNS Microglia: protect CNS cells from disease Ependymal cells: form CSF in ventricles Schwann: produce myelin around PNS
neurons; help to regenerate PNS axons Satellite cells: support neurons in PNS ganglia
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Myelination Axons covered with a myelin sheath
Many layers of lipid and protein: insulates neurons Increases speed of nerve conduction Appears white (in white matter)
Nodes of Ranvier: gaps in the myelin Nodes are important for rapid signal conduction
Some diseases destroy myelin: Multiple sclerosis Tay-Sachs
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Collections of Nervous Tissue Clusters of neuron cell bodies
Ganglion: cluster of cell bodies in PNS Nucleus: cluster of cell bodies in CNS
Bundles of axons Nerve: bundle of axons in PNS Tract: bundle to axons in CNS
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Gray and White Matter White matter: primarily myelinated axons Gray matter: cell bodies, dendrites,
unmyelinated axons, axon terminals, neuroglia Locations of gray and white matter
Spinal cord: white matter (tracts) surround centrally located gray matter “H” of “butterfly”
Brain: gray matter in thin cortex surrounds white matter (tracts)
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Neuron Regeneration Regeneration of PNS neurons
Axons and dendrite in the PNS can be repaired if cell body is intact and Schwann cells functional. These form a regeneration tube and grow axons or dendrites if scar tissue does not fill the tube
Regeneration of CNS neurons Very limited even if cell body is intact Inhibited by neuroglia and by lack of fetal growth-
stimulators
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Organization of the Nervous System Central nervous system (CNS) structures:
Brain Spinal cord
Peripheral nervous system (PNS) structures: Cranial nerves and branches Spinal nerves and branches Ganglia Sensory receptors
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Organization of the Nervous System
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Organization of the Nervous System Peripheral nervous system (PNS) divisions
Somatic (SNS) Sensory neurons from head, body wall, limbs, special
sense organs Motor neurons to skeletal muscle: voluntary
Autonomic (ANS) nervous systems Sensory neurons from viscera Motor neurons to viscera (cardiac muscle, smooth
muscle, glands): involuntary Sympathetic: “fight-or-flight” Parasympathetic: “rest-and-digest”
Enteric nervous system (ENS): “brain of the gut”
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Organization of the Nervous System Peripheral nervous system (PNS),
Enteric nervous system (ENS): “brain of the gut” Sensory neurons monitor chemical changes and
stretching of GI wall Motor neurons regulate contractions, secretions and
endocrine secretions (involuntary)
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Structure and Function of the Nervous SystemInteractions Animation Introduction to Structure and Function of the
Nervous System
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Action Potentials Action potentials = nerve impulses Require
A membrane potential: a charge difference across cell membrane (polarization)
Ion channels: allow ions to move by diffusion from high to low concentration
Leakage channels: allow ions to leak through membrane; there are more for K+ than for Na+
Gated channels Open and close on command Respond to changes in membrane so can generate and
conduct action potentials
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Resting Membrane Potential Typically –70 mV
Inside of membrane more negative than outside Caused by presence of ions:
Inside (more negative) because cytosol has: Many negative ions (too large to leak out): amino acids
(in cellular proteins) and phosphates (as in ATP) K+ that easily leaks out through many K+ channels
Outside (more positive) because interstitial fluid has:
Few negative ions Na+ that does not leak out of cell: few Na+ channels Membrane “pumps” that quickly pump out Na+ that
does leak (diffuse) into cell
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Resting Membrane Potential
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Action Potential Series of events that activate cell membrane
in neuron or muscle fiber An initial event (stimulus) is required
Triggers resting membrane to become more permeable to Na+
Causes enough Na+ to enter cell so that cell membrane reaches threshold (~ –55 mv)
If so, the following events occur: action potential which spreads along neuron or muscle fiber
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Action Potential Depolarizing phase
Na+ channels open as more Na+ enters cell, membrane potential rises and becomes positive (–70 0 + 30 mv)
Repolarizing phase K+ channels open as more K+ leave cell,
membrane potential is returned to resting value (+ 30 0 –70 mv)
May overshoot: hyperpolarizing phase
Typically depolarization and repolarization take place in about 1 millisecond (1/1000 sec)
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Action Potential
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Action Potential Recovery
Levels of ions back to normal by action of Na+/K+ pump
Refractory period (brief): even with adequate stimulus, cell cannot be activated
All-or-none principle If a stimulus is strong enough to cause
depolarization to threshold level, the impulse will travel the entire length of the neuron at a constant and maximum strength.
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Membrane PotentialsInteractions Animations
Membrane Potentials
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Conduction of Nerve Impulses Nerve impulse conduction (propagation)
Each section triggers the next locally as even more Na+ channels are opened (like row of dominos)
Types of conduction Continuous conduction
In unmyelinated fibers; slower form of conduction Saltatory conduction
In myelinated fibers; faster as impulses “leap” between nodes of Ranvier
Factors that increase rate of conduction Myelin, large diameter and warm nerve fibers
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Conduction of Nerve Impulses
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Conduction of Nerve Impulses
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Synaptic Transmission Similar sequence of events occurs at
Synapse (neuron-neuron) Neuromuscular junction (neuron-muscle fiber:
chapter 8) Neuroglandular junction (neuron-gland)
Triggered by action potential (nerve impulse) Components of synapse:
Sending neuron: presynaptic neuron (releases neurotransmitter)
Space between neurons: synaptic cleft Receiving neuron: postsynaptic neuron
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Synaptic Transmission Action potential arrives at presynaptic
neuron’s end bulb Opens voltage gated Ca2+ channels Ca2+
flows into presynaptic cytosol Increased Ca2+ concentration exocytosis
of synaptic vesicles Neurotransmitter (NT) released into cleft NT diffuses across cleft and binds to
receptors in postsynaptic cell membrane
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Synaptic Transmission NT serves as chemical trigger (stimulus) of
ion channels Postsynaptic cell membrane may be
depolarized or hyperpolarized Depends on type of NT and type of postsynaptic
cell 1000+ neurons converge on synapse; the sum of
all of their NTs determines effect If threshold reached, then postsynaptic cell
action potential results
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Synaptic Transmission One-way transmission only because
Only presynaptic cells release NT Only postsynaptic cells have receptors for NT
binding Finally, NT must be removed from the cleft.
Three possible mechanisms Diffusion out of cleft Destruction by enzymes (such as ACh-ase) in cleft Transport back (recycling) into presynaptic cell
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Signal Transmission at the Chemical Synapse
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Neurotransmitters Acetylcholine (ACh): common in PNS
Stimulatory (on skeletal muscles) Inhibitory (on cardiac muscle)
Amino acids Glutamate, aspartate, gamma aminobutyric acid
(GABA), glycine Modified amino acids
Norepinephrine (NE), dopamine (DA), serotonin Neuropeptides such as endorphins Nitric oxide (NO)
Copyright 2010, John Wiley & Sons, Inc.
End of Chapter 9
Copyright 2010 John Wiley & Sons, Inc.All rights reserved. Reproduction or translation of this work beyond that permitted in section 117 of the 1976 United States Copyright Act without express permission of the copyright owner is unlawful. Request for further information should be addressed to the Permission Department, John Wiley & Sons, Inc. The purchaser may make back-up copies for his/her own use only and not for distribution or resale. The Publishers assumes no responsibility for errors, omissions, or damages caused by the use of theses programs or from the use of the information herein.